Conventionally mortar increment containers (MICs) used to contain propellant used by the U.S. Army for 60 mm, 81 mm, and 120 mm projectile propulsion systems are manufactured of a felt fiber, which is composed of nitrocellulose (NC), kraft, resin and various additives—that add to the energy imparted to the projectile. Unfortunately, this manufacturing process involves multiple steps including matting, condensing and pressing fibers, which are labor intensive and relatively costly. Further, it is known that moisture can negatively impact the velocity and range of felt MICs by as much as 5%.
An alternative material to felt MICs, which has been adopted by NATO, is non-porous celluloid (hereinafter celluloid), a material which is not significantly affected by moisture, is easily moldable and is relatively low cost—and which still adds to the energy imparted to the projectile. Celluloid is a class of compounds based upon nitrocellulose, a highly flammable compound formed by nitrating cellulose through exposure to nitric acid, or another strong nitrating agent. Typically, celluloid is composed of 70 to 80 parts nitrocellulose, nitrated to an 11% nitrogen, and about 30 parts camphor, which acts as a plasticizer for the nitrocellulose. The nitrocellulose and camphor are mixed in the presence of solvents, such as ethanol or in a mixer, followed by straining, roll milling and “hiding”. A selected number of “hides” are then blocked at a desired pressure and temperature into a fused block, which is then sliced into sheets at desirable thickness after a conditioning period. Celluloid may contain a number of additives such as dyes and fillers for various applications—more common uses today include guitar picks, ping-pong balls, and some writing and musical instruments.
It is known that typical celluloid combustible cases experience residue issues, as well as, having mechanical strength and embrittlement issues, especially at low temperatures. Of these issues the most troubling is residue, as combustible increment containers used in mortar and artillery propulsion systems must burn cleanly, free of after-combustion residue, to avoid creating an obstruction within the launch tube of the projectile system. Any obstruction within the launch tube can lead to misfires or hang fires which could result in the immediate detonation of the projectile, with significant potential for injury or death of the crew.
Thus there is a need in the art for a relatively low cost, easily moldable, MIC material of manufacture that does not suffer from the wetness or manufacturing problems associated with felt, or the embrittlement, mechanical strength or residue problems associated with celluloid. The subject MIC material should contain an energetic constituent as does the felt fiber or celluloid of the prior art.